In general, an excavator is operated by two main pumps driven by an engine, and a main control valve provided with a plurality of spools for distributing hydraulic fluid pressurized by the two main pumps to a boom, an arm, a bucket cylinder, and a swing motor and recovering the hydraulic fluid therein.
Further, an excavator boom actuating system is configured so that when hydraulic fluid is supplied to a cylinder head side of a boom cylinder by main pumps, the boom ascends, and when hydraulic fluid is supplied to a rod side of the cylinder, the boom descends. The ascending and the descending of the boom is determined according to an operation direction of a boom joystick, and an ascending speed and a descending speed of the boom are determined according to an amount of operation of the joystick.
The boom receives hydraulic fluid in the boom cylinder by one main pump at an initial stage, and when a large flow rate is necessary, the boom receives hydraulic fluid from two main pumps by the main control valve.
In general, a hydraulic system for actuating the boom has very low efficiency, and especially, small flow rate section, in which the boom cylinder is driven by one main pump, has much lower energy efficiency than that of a large flow rate section, in which two main pumps are used. That is, when the boom ascends, a lot of flow loss is generated in the main control valve to a fine manipulation operation section corresponding to approximately ½ of a maximum supply flow rate of the main pump, so that energy efficiency is very low.
Further, energy supplied while the boom ascends is stored in a form of potential energy of the boom, and an amount of regenerable energy of the boom is predicted as approximately 90% of supply energy. However, according to a hydraulic system of an excavator in the related art, most of the regenerable energy of the boom stored in the form of the potential energy of the boom is converted into heat by meter-out control in the main control valve when the boom descends, to be lost when the boom descends.
In a case of the flow rate supplied to the boom cylinder by distributing the flow rate to each actuator during general excavating work, the number of cases where a ratio of the flow rate to a maximum flow rate of the main pump is equal to or larger than a predetermined ratio is small, and in view of power, a case where maximum engine power is completely used is hardly generated. Accordingly, a use of a hydraulic pump motor with a large capacity in order to respond to momentarily increasing power requirement/regenerative power, and a large flow rate is not efficient.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.